[Dwarf-Discuss] .cfi_offset questions

[Jonas H.]

Hi!

I hope this is the right mailing list to ask this kind of question.

I?m working on a edu/research compiler that has yet to be equipped with DWARF info. Unlike GCC/Clang/? it doesn?t have spill slots that are ?constant? within a procedure, i.e. may procedure code like this:

_main:
  ?
  je B
A:
  movl %edi,  -8(%rbp) # EDI spill slot: -8
  ?
  jmp C
B:
  movl %eax,  -8(%rbp)
  movl %edi, -16(%rbp) # EDI spill slot: -16
  jmp C
C:
  ?

Is this supported by the DWARF .cfi_offset directives at all, and if so, how? Generally speaking, what?s the ?scope?/?validity period?/?life span? of such directive?

As an example, if C were to raise an exception and we?d use stack unwinding, how could the unwinder find the old/callee-saved EDI value?

Thanks
Jonas